The invention relates to the means of installing a conventional thermocouple in the interior of the air-gas supply duct of a gas burner, in order for it to provide, at one and the same time:
a) the function of xe2x80x9ccold safetyxe2x80x9d in the case of flame extinction,
b) the function of xe2x80x9chot safetyxe2x80x9d in the case of the injector catching fire, and this without any additional device,
c) a considerably increased operational service-life, thanks to the permanent cooling, by the xe2x80x9ccoolxe2x80x9d air-gas mixture, of this thermocouple which is very highly heated at the level of its elements close to the combustion chamber.
The device is particularly intended for apparatuses, the combustion chamber of which operates in a confined space at high temperatures and nevertheless requires the thermocouple to be positioned at the level of said chamber.
In the present state of the art, thermocouples are component parts of a well-known safety device, the external appearance of which is in the form of a metal sheath, called an external conductor, which terminates, on the side exposed to the heat, in a sensing point in a bulb which forms a sleeve subject to a casing and, at the other end, in an electrical connection to a safety valve.
Inside the bulb sleeve, a segment formed from a specific metal which differs from that of the sleeve of the bulb is welded at its end to the point of the latter. This weld is called a xe2x80x9chot weldxe2x80x9d. This segment is extended, at its other end, by another weld to an insulated conductor wire enclosed in the external conductor tube. This second weld is called a xe2x80x9ccold weldxe2x80x9d. Finally, the internal conductor wire is connected to the outlet of the conductor tube, the connection to the safety valve. Internal conductor and external conductor are thus connected to an electromagnetic coil inside a safety valve. When the sensor point of the bulb is subjected to the heat of a burner, the temperature difference which is established between the hot weld and the cold weld generates a movement of the electrons, and the difference in potential created engenders a continuous micro-current, the electromotive force of which is capable of inducing, at the level of the solenoid of the coil of the safety valve, an electromagnetic field sufficient to maintain, in the attracted position against the electromagnet of said coil, a displaceable core which carries the opening-closing flap of the safety valve. If the sensor point of the thermocouple cools when the production of heat by the burner ceases when it is extinguished, intentionally or not, the difference in potential disappears, and therefore also the electromagnetic field, and the flap, withdrawn by a spring, returns into a closed position. In the present state of the art, this safety device by thermocouple is entirely satisfactory in numerous applications where the configuration of the combustion apparatuses of the gas permits the thermocouple to be positioned so that only the hot weld of the sensor point is exposed to the heat of a flame. This is principally the case in devices with pilot lights or combustion igniters not confined in an environment of excessive temperature. And if the temperature, solely at the sensor point, does not exceed 600xc2x0 C., the service-life of the thermocouple remains within the limits of a normal service-life and does not pose any special problems other than reasonable maintenance when the wear on the sensor point exposed to the oxidising combustion of a naked flame finally destroys the hot weld, thereby preventing the generation of the desired electromotive force.
The situation is not as satisfactory, or even acceptable, in other applications, and in particular new applications, where it is not possible to limit the exposure to heat of the only sensor point of the thermocouple and to contain, moreover, this exposure to heat within a temperature gradient which does not exceed 600xc2x0 C. at the maximum. This is the case, by way of a non-limiting example, with the new combustion chambers of xe2x80x9chigh temperaturexe2x80x9d infrared radiators with metal refractory grilles where the desired configuration necessitates the introduction of the sensor point into a confined combustion chamber to attain temperatures reaching more or less 950xc2x0 C.
In these conditions, not only is the sensor point of the bulb subjected to a temperature exceeding its own limits of 600xc2x0 C. for its service-life, but also the casing to which this bulb is secured is subjected to an unacceptable temperature by conduction. It is inside and at the level of this casing that the xe2x80x9ccoldxe2x80x9d weld is situated. This cold weld, raised to too high a temperature, affects the value of the difference in potential with the xe2x80x9chotxe2x80x9d weld but, above all, subjects the segment between these 2 welds, which is formed from a specific alloy, to inexorable destruction by material loss according to a geometry of xe2x80x9cpencil pointsxe2x80x9d opposed by the summit (in the image of electric arc electrodes), this deterioration continuing until there is a definitive break in the segment at the level of the 2 opposite points.
To this is immediately added, upstream of the casing, the attack on the external conductor which becomes porous and particularly sensitive to oxidation and to the corrosive action of the burnt gases released by the neighbouring combustion.
In these temperature and proximity conditions, the service-life of a conventional thermocouple is reduced very substantially. The efforts used to extend this service-life consist substantially of retarding these destructive effects:
d) reinforcing the volume of the hot weld,
e) treating the surface with a deposit of nickel on the casing and on the portion of the external conductor close to the hot area.
No matter what is done, it must be admitted that these efforts for improvement only relate to the effects and not to the cause.
With reference to hot safety devices, in case of the injector catching fire, known electrical cut-off devices follow the xe2x80x9cmechanicalxe2x80x9d concept, in that this break occurs by a fuse blowing or by opening a thermostatic contact. These devices obviously lead to extra cost with, moreover, the disadvantage of a xe2x80x9cpiratexe2x80x9d shunt always possible by an unaware or careless user.
Therefore, the object of the present invention is:
for cold safety:
f) to treat the cause of the precocious deterioration of thermocouples when these are placed, of necessity, in the very centre of combustion chambers which are raised to a high temperature, reaching more or less 950xc2x0 C.;
for hot safety:
g) to take advantage of the consequences induced by the installation means used in order to treat this cause of deterioration, also to permit the thermocouple to provide the function of hot safety without a xe2x80x9cmechanicalxe2x80x9d electrical cut-off device.
And hence to obtain, overall, with a conventional thermocouple, long use at minimum cost, since it is used to provide, at one and the same time, cold and hot safety functions on apparatuses having a confined combustion chamber which have the advantage of generating high temperatures. These high temperatures are particularly desirable for infrared light emitters in order to obtain electromagnetic wavelengths of between 1.5 and 4 micrometers.
More precisely, the invention comprises a device for installing a thermocouple in the interior of the air-gas duct of a gas burner so as to ensure the functions both of cold safety and of hot safety, said thermocouple comprising an external conductor, which terminates in a sensor point in a bulb forming a sleeve subject to a casing, characterised in that the downstream portion of the external conductor of the thermocouple, on the sensor point side, penetrates to the interior of the inlet duct for the air-gas mixture, in that the sensor point at the end of the bulb is supported on the internal surface of the contact zone of the perforated wall of a diffusion chamber, and in that the installation device comprises a small plate secured transversely to the outlet section of the air-gas duct, said small plate having an appropriately dimensioned surface, to fulfil the function of a localised retardant screen for the air-gas flow, at the level of the contact zone with the internal surface of the diffusion chamber, the base of the casing for supporting the bulb also being supported in the gap of the small guiding and positioning plate, the small plate being subject either to the outlet of the air-gas duct or to the base of the diffusion chamber on the outlet side of the duct.
It is appropriate to demonstrate the operating principle of the present invention by a general presentation. Therefore, the device which is the subject-matter of the present installations and combinations consists of:
h) positioning the thermocouple by causing it to be placed inside the inlet duct of the air-gas mixture, to cause the sensor point to emerge, at the end of the bulb inside the diffusion chamber at the centre of the combustion chamber or, if there is no diffusion chamber, at the outlet of the supply duct in the combustion chamber itself, according to the installation details which will be described later in an additional example, with reference to FIG. 3,
i) guiding the bulb 4 of the sensitive element (sensor point) of the thermocouple introduced into the air-gas mixture supply duct and ensuring its exact positioning, the sensor point coming into abutment against the internal wall of the grille which constitutes the diffusion chamber,
j) creating, by a deflector at the outlet of the tube on the diffusion chamber side, a mini-zone sheltered from the cool and rapid flow of the air-gas mixture not yet ignited.
This mini-zone, which is calmer, compels a small fragment of the grille wall of the diffusion chamber to become red very locally. On the other side of the grille, in fact, the air-gas mixture combusts at the level of the grille in the sheltered mini-zone. It is precisely at this very spot that the sensor point abuts against the reddened grille or only there that the hot weld end is drawn by the necessary and sufficient heat to generate the desired electromotive force and therefore to provide the function of xe2x80x9ccoldxe2x80x9d safety, thanks to the sheltered mini-zone. The temperature ranges, at the sensor point, are confined to around 500xc2x0 C. Upstream of the sensor point and of the bulb, all of the rest of the thermocouple, being in fact ventilated by the cool air-gas flow, sees its temperature settled at a very much lower level than that of the hot weld, this difference reaching several hundreds of degrees centigrade.
Moreover, this ventilation by the new and cool flow shelters it from any contact with the ambient gases of the neighbouring combustion, gases which are even more corrosive when they are extremely hot, and, as a consequence, very damaging for the thermocouple in respect of its service-life. To fulfil the function of hot safety, without an additional mechanical device, in the case of the injector catching fire, it is the heat of this internal combustion itself, in the air-gas duct before its outlet into the diffusion chamber, that is used. This internal ignition then encompasses the external conductor of the thermocouple which runs through the interior of the duct as far as the casing where the cold weld is situated. Since the combustion zone is thus displaced from the burner in the duct, the cold weld heats up while the hot weld cools down. The electromotive force is reduced, and the gas shut-off safety operation is automatic. With the means put into effect according to the invention, this inversion of temperatures is exactly the consequence, for an internal thermocouple, of the injector catching fire. In this eventuality, in fact, since the air-gas mixture ignites in the duct and no longer in the combustion chamber, the casing containing the cold weld becomes hotter than the bulb assembly and the sensor point which contain the hot weld. In a time lapse in the order of 15 to 45 seconds, depending on the features of the component parts used, the electrical cut-off interrupts the firing. The brevity of this abnormal event does not affect the external conductor sufficiently to cause it to deteriorate seriously, and experiments show that about ten accidents of the injector catching fire in succession, and therefore nine maintenance operations of an apparatus to be missed, would be needed to have to replace the thermocouple.